When waterbeds were first popularized, the bags usually comprised two layers of plastic material peripherally butt welded together to form a simple pillow-like configuration. Later, the bags were made in a three dimensional configuration better to fit the frame to avoid abrasion and excessive fatigue.
Whether made as a simple pillow or as a more complex three dimensional structure, bags are less prone to fail if the welds are lap welds rather than butt welds. Thus, butt welds tend progressively to peel apart under internal stress and are not as strong as lap welds which utilize shear forces to resist failure. Moreover, lap welds provide no rim or flange protuberances that must fold to conform to the frame and, consequently, stress the material in so doing.
The most desirable structure is a three dimensional box-like structure that is entirely lap welded. U.S. Pat. No. 3,753,819 to Carlos A. Mollura, entitled TUBULAR FLEXIBLE BAG WITH LAP WELDED ENDS, describes a method by which such a bag may be constructed. A sheet of plastic material is looped to form a tube, the ends being partially lap welded to leave an intermediate slit. The tube is then telescoped over a floating die that has generally rectangular ends. The edges of the tube are folded inwardly to form a marginal flange. The corners of the die are slightly rounded so that bunching and overlapping is avoided. End pieces are placed over the die and welded in place. A one-to-one weld is achieved. The die is removed through the slit which is subsequently closed with the aid of a separator stick, in turn subsequently removed through the fill aperture.
It has become increasingly important to provide so-called "waveless" waterbed bags, that is, bags in which the water provides the conformable support, but does not provide extended wave motion. It has been proposed to utilize internal baffles. The problem is how to weld such baffles in position and subsequently close the mattress by a lap welded process. In U.S. patent application Ser. No. 796,853 of Carlos A. Mollura, entitled METHOD OF MAKING BAFFLED WATERBED, now U.S. Pat. No. 4,141,770, the baffles are made in companion separable parts whereby the baffles can be assembled as the last or penultimate step in the construction process, the separable baffle parts allowing the bag to be telescoped over the floating die as in U.S. Pat. No. 3,753,819. The solution is tedious and expensive.
The primary object of the present invention is to provide a simple and inexpensive method of making a waveless waterbed mattress bag, that is, one with internal baffle structures within the framework of a 100% lap welded three dimensional structure.